This invention relates to antennas for receiving radiated electronic signals, and more particularly to adaptive antenna arrays. Basically, adaptive antenna arrays are utilized to receive desired information signals in a predetermined frequency band while at the same time rejecting undesired interference signals in the same frequency band. To accomplish this, the information signals and interference signals must be received from the different directions. In one typical operating environment, the antenna array is physically mounted in a guided missile; and the information signals are transmitted by the missile controller from one direction while the interference signals are intentionally transmitted by an alien source from another direction. Such interference signals are commonly known as jamming signals.
In the art, various adaptive antenna arrays have been disclosed which attempt to overcome the above described problem. These prior art arrays include LMS (least means squares) arrays, MSN (maximum signal to noise ratio) arrays, SMI (simple matrix inversion) arrays, and RS (random search) arrays. All of these arrays, and the algorithms upon which their performance is based, have been well described in the literature and thus they will not be further described here.
However, each of the above mentioned prior art antenna arrays have deficiencies which the disclosed invention overcomes. For example, the LMS antenna array which currently enjoys the greatest popularity, operates by placing nulls in its reception pattern at locations corresponding to the directions from which the interference signals are received. This is achieved by minimizing the mean squared error between the array sum output and a known reference signal. A problem with this antenna array however, is that with fewer interference signals than antenna elements or with widely separated antenna elements regardless of the number of interfering signals the reception pattern can suffer an unwanted null at the exact location at which the desired information signal is being received. This cannot occur with the disclosed invention.
Further, the LMS antenna array requires a replica of the information signal to be available at the receiver input. This also is not required by the disclosed invention.
The disclosed invention also has advantages in communication systems which use a narrow receive band for tracking the source of the transmitted information signal. With the disclosed invention, the turning on of a new interference signal while the information signal source is being tracked will not cause lock of track to be broken. By contrast, with an LMS adaptive array, when the new interference signal is turned on, altered weights could deflect the main beam off the proper track angle. And if system range were great, the loss in signal accompanying the beam deflection might not be tolerable.
Therefore, it is one object of the invention to provide improved apparatus for separating desired information signals from undesired interference signals.
Still another object of the invention is to provide an improved method of separating desired information signals from undesired interference signals by an adaptive antenna array.